Comparative optical properties of macerals and statistical evaluation of mis-identification of vitrinite and solid bitumen from early mature Middle Devonian – Lower Mississippian New Albany Shale: Implications for thermal maturity assessment

Abstract

Vitrinite reflectance (VRo in %) is used routinely to quantify thermal maturity in sedimentary basins. Reflectance and fluorescence of other macerals such as solid bitumen and amorphous organic matter (AOM) can provide an independent means to assess thermal maturity and hydrocarbon generation potential. However, similarity in petrographic characteristics of these macerals, solid bitumen and vitrinite in particular, often causes difficulties with their identification and, consequently, reflectance can be measured on misidentified particles, making reported VRo values unreliable. The purpose of this study is to compare reflectance values of various macerals in early mature shales and to evaluate the implications of misidentifying solid bitumen and vitrinite for assessing thermal maturity. To address this issue, 15 organic-matter-rich samples from the Middle Devonian/Lower Mississippian New Albany Shale from a corehole in Daviess County, Indiana, were selected. These samples were chosen because they had distinct and easily identified AOM, solid bitumen, vitrinite, and inertinite particles, allowing for statistically relevant comparisons.VRo values (0.57 to 0.65%) of the studied sample suite cover the early mature stage, and expressed no trend with depth. In comparison, reflectance values of solid bitumen (BRo) and AOM (AOMRo) from the same samples are lower, and range from 0.44 to 0.52% and 0.27 to 0.31%, respectively. These differences are accompanied by corresponding differences in chemistry of macerals as demonstrated by the micro-FTIR technique. Specifically, compared to vitrinite and inertinite, solid bitumen shows lower aromaticity, and compared to AOM and alginite, it exhibits shorter aliphatic chains. Reflectance was observed to vary systematically; samples having higher VRo also feature elevated solid bitumen and AOM reflectance values. The relationship between vitrinite and solid bitumen can be expressed by the following equation: vitrinite reflectance equivalent (VRoE in %)=(0.83×BRo)+0.22, whereas for vitrinite and AOM, VRoE=(0.84×AOMRo)+0.38. Statistical evaluation of the differences in reflectance values caused by maceral misidentification indicates that in extreme cases, when a petrographer cannot distinguish between vitrinite and solid bitumen, the reflectance can be shifted by 0.06–0.09%. For this set of samples, such a difference could shift maturity assessment from early mature to immature. In more common cases, when the analyst can distinguish between the macerals but has difficulties with their overlapping reflectance interval, the reflectance difference that results from misidentification is only within a 0.0–0.02% range. Therefore, if the level of uncertainty in maceral identification can be determined, the calculation of VRoE values from measured BRo, can reduce inaccuracy in VRo values for interpreting the thermal histories of sedimentary basins, which, in turn, is essential for the assessment of oil and gas resources and for building a successful exploration model.